Well-test interpretation in case of behind-the-casing crossflow

Abstract

Abstract This study considers the potential use of well-testing for investigating cross-flows in production wells. Using a numerical model, the authors detected pressure behavior patterns which may become informative characteristics for diagnosing a cross-flow. To increase diagnostic reliability, the optimization of measurement technologies and interpretation methods to manage pressure behavior pattern characteristics are substantiated. These testing techniques make it possible to distinguish behind-the-casing fluid movement where background interference is present. One of the most interesting areas of application of the results obtained in this study is monitoring unsteady fracture creation in injection wells. Such fractures occur when the injection pressure exceeds reservoir fracturing pressure, and they break down shale layers between formations. When pressure is reduced, the fractures close up, and cross-flow stops. That is why the derivative behavior during the injection cycle significantly differs from that of the static state. Numerous examples illustrate that well testing is considerably more successful when combined with downhole logging. Introduction The conventional purpose of well testing is to investigate filtration characteristics, energy properties, formation geometry, and perfection of well penetration. 1,2,3,4,5,6,7, etc. Until now, the influence of interlayer behind-the-casing cross flows on well testing results has been considered an obstacle to reliable determination of formation parameters. Cross flow investigation, however, is an important independent task. The primary obstacle for its solution is that cross flow effect is difficult to identify against a background of numerous disturbances 8,9,10,11,12. To evaluate the informative value of well testing for this task, the authors used numerical modeling. Two types of cross-flows were considered: a cross-flow through permeable cement stone, and through a hydraulic fracture. The model included the following parameters: filtration characteristics, degree of perfection of formation penetration, behind-the-casing flow intensity, shape and size of the cross-flow channel, characteristics of well operation regimes, etc. Analysis of the modeling results enabled us to identify the unique features of well pressure behavior related to fluid and gas movement behind-the-casing. Cross-flow diagnostics based on these characteristics, however, is not always unequivocal. Similar well testing results may relate to other causes as well. The main idea of the work presented here is as follows: In order to increase the reliability of well testing interpretation, well research must be conducted in such a way that the primary effect under study (behind-the-casing flow in this instance) is manifested as distinctly as possible against the background of other influencing factors. Existing injection wells are among the most promising objects for implementing such an approach. It is well known that water-injection wells are frequently fractured due to the high bottom hole pressure and consequently have a negative skin effect. These fractures heal upon well shutdown. An induced fracture may connect adjacent formations, which is the primary cause of cross-flow initiation. The cross-flow intensity will depend on pressure. Filtration parameters detected by well testing in such unsteady systems will depend on the mode of well operation. This is primarily related to changes in effective thickness participating in operation. When integrated research involves start-up, flow rate change, and shutdown cycles, then interpretation of well testing results must change appropriately from one cycle to the other. This effect, along with a specific configuration of pressure derivative curve, is an additional cross-flow diagnostic feature.